1. Field of the Invention
The invention relates to numerically controlled apparatus and methods for grinding and particularly as applicable to cutting tools.
2. Description of the Prior Art
While the invention has a broad application to grinding a wide variety of tools or parts having edges and surfaces whose relative position and shape or form can be defined mathematically, the prior art and the invention will be discussed primarily in connection with grinding of a ball nose end mill as an illustrative case and because of the complexity of such a grinding operation. From such discussion, those skilled in the art will appreciate that many, if not all, of the same basic problems which have affected the grinding of a ball nose end mill have also affected the grinding of other tools of equal or simpler complexity and with respect to which the invention is equally applicable. Also, such explanation will indicate how the invention may be applied to other grinding operations such as encountered in grinding machine parts, e.g., hydraulic valve spools, cams, crush form dressing rolls, and the like, or as encountered in engraving operations.
End mills in general and ball nose and mills being used for illustration, as now manufactured, are known to have uneven flute indexing that is not equally spaced and to have varying helix angles which can be attributed both to the mill of the flutes as well as to distortion caused in heat treating the metal. Grinding of a ball nose end mill has historically represented a formidable and challenging problem for which many solutions have been sought over a long period of years.
According to early prior art methods of grinding cutting tools, in particular ball nose end mills, the gash was manually ground resulting in the gash having an undesirable negative rake and also resulting in nonuniformity from one flute to the next and from one tool to the next. The O.D. relief was ground using a finger that followed the flute whereby any imperfections in the flute were carried over and produced in the O.D. relief. This grinding technique was necessarily a dry operation, i.e. without a lubricant or coolant fluid, since it was based upon a "see and hear" technique. By this is meant that the operator had to see the tool engage the wheel, had to see the finger engage the tool and had to hear the wheel touch the metal. Also, such technique required that the direction of the grinding wheel edge be towards the cutting edge or surface being ground. This caused heat cracks and burs.
An improvement over the time-honored manual grinding technique is described in commonly-owned U.S. Pat. No. 3,680,263. This patent teaches a grinding machine for grinding the gash in a workpiece having helical fluting and forming end cutting teeth on such workpiece. The workpiece and grinding wheel are held on independent, manually adjustable support systems. The grinding wheel support system provides for no coordinated axial movements though the workpiece support system mechanically couples two axial movements. The grinding wheel support system, however, provides for independently manual positioning on two linear axes and one rotative axis. The workpiece support system allows independent, manual positioning on two linear axes in addition to rotative positioning of the workpiece itself about its longitudinal axis. Thus while this patented grinding machine has made provision for relative positioning of the tool and grinding wheel with reference to a relatively large number of reference axes, the complete geometry has not been obtained primarily because all of the necessary axis motions for the workpiece and grinding wheel positions were not able to be positioned simultaneously and were necessarily made manually and by visual observation. It has been found that while this was an improvement in the state of the art, the lack of uniformity from one tool to the next was still a factor and cutters were not ground to their optimum possibilities nor was it possible to grind the tools complete with the apparatus described in the patent. Also, there has been the continuing practice of the "see and hear" technique, previously explained.
Other improvements directed to automated grinding of cutting tools are illustrated in U.S. Pat. Nos. 3,680,262; 3,719,459; 3,813,823, and 3,816,995. These patents generally teach systems for grinding end mills by machine controlled operations; thus, in some instances, reducing the amount of skill required to duplicate work from one tool to the next. U.S. Pat. Nos. 3,680,262 and 3,813,823, for example, teach a system having three separate stations at which different operations are performed on the tool under automatic control and with use of a coolant. A first station is the radius/O.D. grinder. The second station is the gasher. The third station is the reliever. Simultaneous movement of two reference axes is achieved with a cam-follower arrangement. A primary drawback of this system, however, is that each end mill must be manually loaded and unloaded into each of the three stations before grinding is completed. The repositioning of the end mill and three different tool holders necessarily results in a nonuniformity among the finished end mills. Furthermore, the three separate grinding machines have to be set up manually for each size cutter and separate grinding wheels, work holders, and the like, must be provided for each machine.
U.S. Pat. No. 3,719,459 also refers to an automatic grinding operation. However, the grinding machine of U.S. Pat. No. 3,719,459 is primarily directed to grinding the end faces only as distinct from grinding all of the required clearance surfaces and cutting edges as with the present invention. U.S. Pat. No. 3,816,995 improves on U.S. Pat. No. 3,719,459 but the apparatus of both patents is severely restricted in the number of controlled reference axes and in the number of operations that can be completed automatically at one station and by inherent limitations of the hydraulic-air logic control. Furthermore, all of the previously-mentioned prior art is handicapped by the inability to move a relatively large number of reference axes simultaneously and independently under machine control. In contrast, the present invention is to a great extent directed to means for providing independent and simultaneous movement of a large number of reference axes under numerical control and according to precise, mathematically defined conditions.
A further and more recent improvement in an automatically controlled tool grinder was achieved in the numerically controlled grinder sold and identified by the trademark legend "HS-1 Universal Grinder" by S. E. Huffman Corporation, South Main St., Clover, S.C., 29710, assignee of the present invention. The HS-1 grinder was introduced in 1975 and relative movement of the grinding wheel and tool with respect to seven reference axes moving relatively, independently and simultaneously was achieved. However, while the HS-1 grinder represented a significant advance in the prior art, it has been found that an even greater number of reference axes must be moved independently, relatively and simultaneously in order to achieve the accuracies required in modern tool grinding operations which the HS-1 could not do. Also, the HS-1 grinder had limited ability to spin or rotate the A axis because of limitations of control. This gave limitations on ability to perform grinding of high helix, tapered cutters, ball nose end mills and grinding of plain diameters. While the HS-1 grinder allowed the workpiece to spin, this could be achieved only for a limited time and at a limited speed. Thus, it becomes desirable to provide an improved grinder as with the present invention, capable of spinning the workpiece without such limitations in time and speed to allow grinding operations such as grinding of bar stock or for grinding high helix cutters.
The HS-1 grinder also represented an advance over the prior art in providing a type of coolant system under numerical control. However, the HS-1 grinder coolant system lacked the ability to vary the quantity of coolant and basically required the same quantity of coolant to flow whenever coolant was being directed towards the tool being ground in a particular grinding operation. The HS-1 grinder has thus demonstrated a need for greater versatility in the manner in which the coolant can be directed and controlled with respect to both timing and direction as well as quantity and in relation to particular phases of the overall grinding operation. Also of interest to the present invention is the fact that the HS-1 grinder provided a system for compensating for wheel wear based on automatic gauging of the wheel diameter and, while considered, a feedback for numerical control wear correction was not achieved. However, this experience has dictated a need for an improved numerically controlled wheel wear compensation system which does not require wheel gauging during the grinding operation but nevertheless provides for automatic numerically controlled wheel wear compensation.
In other respects, the HS-1 grinder, as compared to the present invention, utilized hardwired controls which were limited to the number of axis controllable by one control and to moving three axes simultaneously. The type of controls used with the HS-1 grinder did not provide for storage of programs, memory, use of a CRT tube for program display, editing, or making of tapes, and did not allow for simultaneous movement of five, six, seven, eight, and up to ten axes simultaneously as with the present invention. In the HS-1 grinder control system, it was not possible, for example, for the grinding wheel to be moved up and down with reference to the X, Y, and two rotary axes simultaneously which made the grinding of all cutters, and particularly ball nose end mills, complex as compared to the present invention. Full utilization of the U axis was limited since movement of the U axis simultaneous with the A axis could not be achieved.
Therefore, it becomes an object of the present invention to provide a grinding method for grinding cutters, particularly cutting tools, whereby the entire grinding operation may be performed at one station with one grinding wheel, with the tool being held in the same work holder throughout the entire grinding operation and with all of the grinding steps being achieved under automatic numerical control with a high degree of repeatable precision. A further object of the invention is to provide a grinding method which allows the size and type of the cutting tool being ground to be changed without requiring a new machine setup. A further object is to provide a grinding method that allows resharpening of cutters to exact tolerance, with speed and ease and under massive coolant. Another object of the present invention is to provide a grinding method for grinding a complex workpiece requiring simultaneous movement of multiple axis including 5, 6, 7, 8, e.g., a ball nose end mill, and at the same time allow for automatic wheel wear compensation and automatic wheel size compensation. A further object is to provide a method wherein the grinding wheel can be trued or dressed using the simultaneous motion of two or more rotary axes in conjunction with simultaneous movement of two or more linear axes. Another object is to provide a grinding machine capable of making a complex workpiece complete from softened or hardened bar stock. Further, an object is to achieve such a complex workpiece by use of one grinding wheel or by use of multiple grinding wheels on the same end of the wheel spindle or on both ends of the wheel spindle. The foregoing and other objects will become apparent as the description proceeds.